1. Field of the Invention
The invention relates to a device for sealing the gap between the moving blades and the casing, designed with a conical contour, of a turbomachine, the moving blades being provided with encircling shroud plates which, while forming radial gaps, make a seal against the casing, which is provided with sealing strips.
2. Discussion of Background
Such devices are known. They form a smooth or a stepped half labyrinth having entirely radial gaps. Such a seal is shown in FIG. 2, which is to be described later.
As a result of the better efficiency and the greater reliability, this type of gap seal is in the meantime already being used for the moving blades of the penultimate stage of condensing steam turbines. The mechanical requirements here, at circumferential speeds of 450 m/sec, are quite high, whereas the thermal conditions, at about 90.degree. C., are modest. The geometrical requirements are problematic: on the one hand, on account of the pronounced conicity, which leads to deep cavities of the known sealing device in the casing wall; on the other hand, on account of the large differential expansions between rotor and casing, which lead to wide cavities with the abovementioned half labyrinths.
The large cavity formed in this case in the inlet region of the seal produces an unfavorable cross exchange of flow material with the main flow in the blade duct. This cross exchange is encouraged by the exceptionally large fluctuation of the pressure difference between two adjacent blades in the plane of the leading blade edge. In addition, a pronounced vortex is stimulated in this region by the main flow and the side wall of the shroud band.
Less effective is the half labyrinth having the sealing strips with which the casing is provided and which make a seal against the encircling shroud band. This is because, under the existing conditions, the operating clearance must have a size of about 1/3 of the free chamber height. Even a plurality of sealing strips are therefore not much more effective than a single sealing strip.
Finally, the large cavity in the outlet region of the seal also permits an undesirable cross exchange with the main flow in the blade duct, since here, too, the pressure difference between the adjacent blade tips is subjected to large fluctuations. In addition, the guidance of the main flow is completely lost in this region.
In addition, the large vortex space which is formed behind the outer sealing strip and produces considerable dissipation of the outlet-side gap flow is of disadvantage in the case of these seals.